Cytoskeleton

Cytoskeleton Overview

• Composed of a 3-dimensional network of protein filaments within the cytoplasm.
• Adapts to form either dynamic or stable structures, crucial for cellular integrity and function.

Intermediate Filaments

• Non-dynamic structures providing structural support and reinforcement to cells.
• Provide scaffolding particularly in cells demanding structural integrity.
• Types include:
  • Keratin filaments (Type I and II): Predominantly in epithelial cells.
  • Vimentin and Vimentin-related filaments (Type III): Found in connective tissue, muscle, and glial cells.
  • Neurofilaments (Type IV): Located in neuron axons.
  • Nuclear lamina (Type V): Found within the nuclear envelope.
• Mutations in keratins can cause conditions like Epidermolysis Bullosa Simplex, resulting from mutations in Keratin -5 and -14, leading to fragile skin and blisters due to compromised anchoring to the basal lamina.

Microtubules

• Composed of rigid hollow rods that are highly dynamic.
• Serve multiple functions including:
  • Intracellular transport, organization, and maintaining cell shape.
  • Mitotic spindle formation during cell division (mitosis).
  • Making up structures such as cilia and flagella.
• Structurally organized through tubulin heterodimers, binding in a head-to-tail manner with GTP binding.
• Exhibit dynamic instability, rapidly switching between polymerization and depolymerization, essential for cellular functions.
• Centrosomes act as organization centers where microtubules grow toward the plus end; they contain y-tubulin to initiate assembly.
• Motor proteins facilitate intracellular transport:
  • Kinesins: Motor proteins that move towards the plus end.
  • Dyneins: Motor proteins that move towards the minus end.

Actin Filaments (Microfilaments)

• Form thin, flexible structures regulated by actin-binding proteins with two forms: G-actin (globular) and F-actin (filamentous).
• G-actin exists in three isoforms, with different distributions among muscle and non-muscle cells.
• Functions include:
  • Muscle contraction facilitated by myosin.
  • Formation of cell surface extensions like microvilli for absorption and secretion.
  • Providing tension and stabilization to the plasma membrane.
  • Vital for cytokinesis during cell division.
• Elongation occurs through ATP-bound monomers adding to the plus end while ADP-bound monomers dissociate from the minus end.
• Actin structures can be bundled for projections (microvilli) or organized in networks for membrane support.
• Key regulatory proteins:
  • Formins: Promote linear actin formation.
  • Arp2/3 complexes: Initiate branched actin formation.
  • Tropomyosin: Stabilizes actin filaments, especially in muscle cells.
  • Spectrin: Provides cytoskeletal support in the cell cortex, crucial for red blood cell membranes.
  • Dystrophin: Links actin to cell membranes in muscle cells; loss causes muscular dystrophies such as Duchenne Muscular Dystrophy.

Microvilli

• Actin-based structures that increase cell surface area to enhance nutrient absorption and secretion.
• Anchored to a terminal web, a dense region of the cytoskeleton providing stability.

Cytoskeleton Overview

• Composed of a 3-dimensional network of protein filaments within the cytoplasm.
• Adapts to form either dynamic or stable structures, crucial for cellular integrity and function.

Intermediate Filaments

• Non-dynamic structures providing structural support and reinforcement to cells.
• Provide scaffolding particularly in cells demanding structural integrity.
• Types include:
  • Keratin filaments (Type I and II): Predominantly in epithelial cells.
  • Vimentin and Vimentin-related filaments (Type III): Found in connective tissue, muscle, and glial cells.
  • Neurofilaments (Type IV): Located in neuron axons.
  • Nuclear lamina (Type V): Found within the nuclear envelope.
• Mutations in keratins can cause conditions like Epidermolysis Bullosa Simplex, resulting from mutations in Keratin -5 and -14, leading to fragile skin and blisters due to compromised anchoring to the basal lamina.

Microtubules

• Composed of rigid hollow rods that are highly dynamic.
• Serve multiple functions including:
  • Intracellular transport, organization, and maintaining cell shape.
  • Mitotic spindle formation during cell division (mitosis).
  • Making up structures such as cilia and flagella.
• Structurally organized through tubulin heterodimers, binding in a head-to-tail manner with GTP binding.
• Exhibit dynamic instability, rapidly switching between polymerization and depolymerization, essential for cellular functions.
• Centrosomes act as organization centers where microtubules grow toward the plus end; they contain y-tubulin to initiate assembly.
• Motor proteins facilitate intracellular transport:
  • Kinesins: Motor proteins that move towards the plus end.
  • Dyneins: Motor proteins that move towards the minus end.

Actin Filaments (Microfilaments)

• Form thin, flexible structures regulated by actin-binding proteins with two forms: G-actin (globular) and F-actin (filamentous).
• G-actin exists in three isoforms, with different distributions among muscle and non-muscle cells.
• Functions include:
  • Muscle contraction facilitated by myosin.
  • Formation of cell surface extensions like microvilli for absorption and secretion.
  • Providing tension and stabilization to the plasma membrane.
  • Vital for cytokinesis during cell division.
• Elongation occurs through ATP-bound monomers adding to the plus end while ADP-bound monomers dissociate from the minus end.
• Actin structures can be bundled for projections (microvilli) or organized in networks for membrane support.
• Key regulatory proteins:
  • Formins: Promote linear actin formation.
  • Arp2/3 complexes: Initiate branched actin formation.
  • Tropomyosin: Stabilizes actin filaments, especially in muscle cells.
  • Spectrin: Provides cytoskeletal support in the cell cortex, crucial for red blood cell membranes.
  • Dystrophin: Links actin to cell membranes in muscle cells; loss causes muscular dystrophies such as Duchenne Muscular Dystrophy.

Microvilli

• Actin-based structures that increase cell surface area to enhance nutrient absorption and secretion.
• Anchored to a terminal web, a dense region of the cytoskeleton providing stability.

Learn about the 3-dimensional network of proteins in the cytoplasm, including intermediate filaments, their characteristics, and types.